1. Field of the Invention
This invention relates to optical measurement and inspection systems, and more specifically, to an optical inspection head and system in which light reflected from a detection optical path is isolated from the illumination and other detection optical paths.
2. Background of the Invention
Optical surface inspection systems are in common use in industry for both analysis and manufacturing test operations. The optical heads used to provide measurements when scanning a surface may combine multiple types of detection. For example, U.S. Pat. No. 7,671,978, issued to the inventors of the present application, discloses optical heads that include both an interferometer and a scatterometer channel. In other applications, single channel systems are used.
Dark field detectors are sensitive to stray light sources and leakage along the optical path. In particular, scattering detectors or scatterometers, are extremely sensitive to parasitic light originating in so-called “ghost images” in the optical system, and to reflection and re-scattering of ambient light. Light reflecting from the dark-field detection subsystem, or an additional bright field detection subsystem can re-enter the optical measurement system and enter (or re-enter) the dark-field detection channel via reflections from optical system components such as lenses and beam-splitters, and also potentially from the surface under inspection.
Therefore, it would be desirable to provide a dark field scattering detection system that prevents light that enters a detection subsystem from being reintroduced to the optical inspection system.